c++: __is_trivially_xible and multi-arg aggr paren init [PR102535]

c++: __is_trivially_xible and multi-arg aggr paren init [PR102535]

is_xible_helper assumes only 0- and 1-argument ctors can be trivial, but
C++20 aggregate paren init means multi-arg ctors can now be trivial too.
This patch relaxes the relevant early exit check accordingly.

	PR c++/102535

gcc/cp/ChangeLog:

	* method.c (is_xible_helper): Don't exit early for multi-arg
	ctors in C++20.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/is_trivially_constructible7.C: New test.